Solid state photodetectors made with integrated circuit techniques, e.g. PIN, (p-i-n) diodes, are very popular. They are extensively applied to various optical devices such as optical sensors, optical receivers and photocouplers. Photodetectors are widely used in integrated photo-electronics, fiber optic communications, optical interconnection, etc.
For example, the backbone links on the Internet are fiber optic cables that require photodetectors at each receiving location to receive the signal transmitted.
Similarly, fiber optic cables requiring photodetectors are extensively used in telephone systems.
Given the constant pressure in integrated circuit fabrication to reduce costs, it is not surprising that various schemes have been attempted to produce photodetectors at lower cost.
FIG. 1 shows in cross section a typical trench photodetector. On the left, a trench has been etched in silicon substrate 10 to a depth indicated by bracket 90. P-type dopant has been diffused out of the trench into the substrate, forming p-type region 112. The trench has been filled with P+ polysilicon 115, forming a conductive path to ohmic contact 110, illustratively silicide, which is connected to ground.
On the right, a counterpart N-type doped area 132 has been formed and an N+ polysilicon area 135 fills the trench up to suicide 130, connected to a positive voltage. A cap layer 120 has protected the top of the substrate 10 during previous processing steps.
Lightly doped or intrinsic region 150 receives incident light and generates electron-hole pairs. The electrons so generated in response to incident light are attracted to the positive terminal. The flow of electrons may be detected by a number of well-known methods that are known in the art.
The trenches have been etched to a depth 90 that is preferably about the same as the penetration depth of the incident light. As is known, light will penetrate silicon to a depth that depends on the wavelength of the light, among other things. Illustratively, light of wavelength 845 nm will penetrate to a depth of about 15 to 20 microns, since silicon has a relatively low absorption coefficient.
Etching trenches is a slow and therefore expensive process. A trench depth greater than the penetration depth brings no additional benefit and is a waste of money. A trench depth shallower than the penetration depth will depend on a cost/benefit tradeoff. If the intensity of the incident light is great enough that an adequate signal may be obtained at a trench depth less than the penetration depth, then the shallow depth may be economically beneficial.
FIG. 2 illustrates a top view of a prior art photodetector. At the top of the Figure, a p-contact 20 has been formed connecting a number of P-type trenches 22-1 through 22-n in a standard layout in which the contributions from several trenches having the same dimension are added. At the bottom, a counterpart n-contact 30 connects the N-type trenches 32-1 through 32-n.
The n- and p-type trenches are preferably spaced in a tradeoff between greater efficiency in intercepting photons and response time of the device.
FIG. 3 shows in cross section a portion of the multi-element photodetector of FIG. 2, in which the n-type trench electrode 235 has a diffused area 232 formed in the intrinsic silicon substrate 10. Similarly, the P-type electrode 215 has a p-type diffused area 212.
This Figure also shows that the width of the two types of trench is the same, illustratively the minimum width permitted by the lithography (or by the technology of trench etching).
A drawback of this prior art arrangement is that it takes two mask levels to form (and fill) the p and n trenches.
A first method that has been used in the prior art is:    Form deep trenches with a first hardmask and first mask;    Remove the first hardmask;    Fill the trenches with a sacrificial oxide such as BSG;    Deposit a second hardmask;    Remove the BSG from every other trench by using a second mask;    Fill the empty (alternating) trenches with a first type of polysilicon;    Planarize the first polysilicon by a technique such as chemically mechanical polishing (CMP);    Remove the second hardmask;    Remove BSG from the other trenches;    Fill the empty trenches with a second type of polysilicon (opposite polarity);    Planarize the second polysilicon by a second CMP step; and    Form contacts to the two polysilicon electrodes.
This first method requires two masks, which are complex and costly. In addition, since this first method requires two masks, there is a mis-alignment issue. A hardmask must be deposited, patterned, and removed twice. The sacrificial oxide has to be removed from the deep trench twice. The n and p trenches have to be filled with two types of doped polysilicon in two separate steps. Polysilicon has to be planarized twice in two CMP steps.
A second method requires two deep trench steps:    Form the first type of deep trenches with a first hardmask and first mask;    Remove the first hardmask;    Fill the trenches with a first type of polysilicon;    Planarize the first polysilicon by a first CMP;    Form the second type of deep trenches with a second hardmask and second mask;    Remove the second hardmask;    Fill the trenches with a second type of polysilicon;    Planarize the second polysilicon by a second CMP; and    Form contacts to the two polysilicon electrodes.
This second prior art method has the drawback that it requires two deep trench steps. Forming deep trenches is very slow and thus is an expensive process requiring considerable process time and significant cost. In addition, this second method requires two masks, which raises the mis-alignment issue. It also requires two hardmask deposition, patterning, removing steps and two polysilicon CMP steps. The n and p trenches have to be filled with two types of doped polysilicon in two separate steps. Polysilicon has to be planarized twice in two CMP steps.
Moreover, polysilicon is used in prior art methods. Two deposition processes are required in order to fill the n-type trenches with n-type polysilicon and the p-type trenches with p-type polysilicon. Furthermore, as the trenches become narrower and narrower as technology advances, the resistance of polysilicon in deep and narrow trenches increases, slowing the response of the photodetector.